1. Field of the Invention
The present invention relates to a cathode active material for a lithium secondary cell used in a cellular phone, more particularly, to a cathode active material for a lithium secondary cell capable of manufacturing the lithium secondary cell having excellent electrochemical characteristics, a high capacity and a long lifetime by using diadochite, and a method for manufacturing the same.
2. Description of the Prior Art
In recent, as an electronic technique has been actively developed, miniaturization and high functionality of new portable electronic machines such as video cameras, personal computers, portables phone and the like have been rapidly accomplished. Accordingly, the secondary cell used in the small-sized electronic device needs high energy density. In particular, the amount of the lithium secondary cell having a charging/discharging function and a high energy density is remarkably increased and the research for improving the performance thereof has been actively progressed widely.
In case of lithium secondary cell, an active material, which can insert and extract a lithium ion, is able to be used as an anode and a cathode and a space between the anode and the cathode is filled with an organic electrolyte or a polymer electrolyte within which lithium ions can be moved. At the charging, the electric energy is stored by the oxidation reaction while the extraction of the lithium ion is progressed, and at the discharging, the electric energy is generated by the reduction reaction.
The lithium metal was used as the anode active material, but at present it is mainly used only in the primary cell, because of the dangerousness such as explosion due to the formation of dendrite lithium. In order to overcome the shortcoming of forming the dendrite, a lithium-metal alloy is considered as a substitute for the lithium. However, the lithium-metal alloy has a problem that the volume thereof is extremely changed at the charging/discharging. In the electrochemical reaction, the change of volume and weak mechanical characteristics increase the irreversible capacity. Therefore, presently, a carbon is used as the anode of the lithium secondary cell commercially available, the carbon has high capacity and low oxidation/reduction potential compared with a metal oxide, a sulfide, and a polymer, and is stable in the structure and has an excellent charging/discharging lifetime.
Now, LiCoO2 is generally used as the cathode active material. However, because the cobalt is very expensive, the research for using LiNiO2, LiMn2O4, LiNi1-xCoxO2, or V2O5 or the like as the cathode active material has been progressed. Here, LiNiO2, and LiNi1-xCoxO2 have a difficulty in the synthesis thereof and a problem in the stability, and LiMn2O4 has a problem that the capacity is lastingly reduced at charging/discharging, and V2O5 has poor voltage characteristics. Therefore, in order to use the cathode active material capable of substituting LiCoO2, a lot of the researches are necessary.
The research for the development of a new cathode active material has been attempted until now. For the first time, the research on LiFePO4 used as the cathode active material was reported in 1997. Since LiFePO4 has excellent flatness profile of voltage, high average potential of 3.4 V and theoretical discharging capacity of 170 mAh/g, it has taken notice [see, A. K Padhi et al. J. Electrochem. Soc. 1188, 1997, and J. B Goodenough et al. U.S. Pat. No. 5,910,382]. Early in the research, LiFePO4 showed the reversible capacity of about 120 mAh/g under the very slow current density.
Recently, the research for the LiFePO4 composite capable of showing the capacity equal to and more than 150 mAh/g in a high rate of the charging/discharging by improving greatly the electric conductivity by means of carbon coating and metal ion doping is announced [See F. Croce et al. Electrochem. Solid-State Lett. 5, A47, 2002 and N. Ravet et al. EP 1049182 A2]. However, synthesis of LiFePO4 composite for improving the electric conductivity has a problem that various complexmical steps are added to the conventional synthesis steps and a difficulty in commercialization, since the synthesis is accomplished at a refined reduction atmosphere. Also, the requirement of high capacity for the portable information communication has not been satisfied with the theoretical capacity of 170 mAh/g.